(1) Field of the Invention
The invention relates to an apparatus for assisting in ventilating the lungs of a patient who cannot sustain enough respiration due to, for example, sleep apnea syndrome.
(2) Description of the Related Art
A number of devices are provided for assisting in ventilating the lungs of a patient who cannot sustain enough respiration due to, for example, sleep apnea syndrome. U.S. Pat. No. 4,655,213 describes an apparatus for treatment of sleep apnea syndrome by providing continuous positive airway pressure (CPAP) to maintain the airway of the patient in a continuously open state during sleep. However, the apparatus of U.S. Pat. No. '213 cannot assist the ventilation of the lungs of a patient sufficiently, since the described device provides the continuous positive airway pressure even during the expiration phase.
In order to solve the above problem, a ventilator described in Japanese Unexamined Patent Publication (Kokai) No. 53-16238 supplies respiratory gas to a patient at predetermined high and low pressure alternatively at a predetermined time interval. However, the ventilator of JPP '238 also cannot assist the ventilation of the lungs of a patient sufficiently, since it provides high and low pressure respiratory gas at the predetermined time interval while the respiratory cycle of a patient is not constant.
Japanese Examined Patent Publication (Kokoku) No. 50-38435 describes a respiration assisting device which detects the pressure of respiratory gas within a conduit between a respiratory gas source and a mask on the patient, and provides predetermined high and low pressure alternatively based on the detected pressure. On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 3-222963 and Japanese Unexamined Patent Publication (Kokai) No. 4-231067 describe ventilators which control the flow of respiratory gas to a patient by detecting the flow rate so as to provide a predetermined high pressure when the detected flow rate to the patient higher than a predetermined reference flow rate, and predetermined low pressure when the detected flow rate is lower than the reference flow rate. A patient using such an apparatus will feel discomfort since the pressures required for the inspiration and expiration phase are not constant high and low pressure, and drastically change during the respective inspiration and expiration phase as well as the transition phase therebetween. Furthermore, in order to change the pressure or the flow rate, the patient must make a considerable expiratory effort against the respiratory gas flow from the respiratory gas source.
European Patent Publication No. 0,452,001 A2 describes a ventilator which controls the pressure of the respiratory gas such that the delivered pressure is directly proportional to the effort of the patient in consideration with the elastance and the flow resistance of the respiratory organ of the patient. The ventilator can control the pressure to the respiratory system including the ventilating apparatus. The ventilator also controls the flow rate of the respiratory gas and the pressure within the conduit between the respiratory gas source and the mask on the patient based on the detected flow rate. Thus, there is the same problem as the above-mentioned prior art, that is, patient must make a considerable expiratory effort against the respiratory gas flow from the respiratory gas source.
Japanese Unexamined Patent Publication (Kokai) No. 4-263876 described a ventilator which controls the respiratory gas flow to the patient by using closed-loop control or feedback control. In general, a ventilator which includes a respiratory gas source, a mask on a patient, a conduit between the respiratory gas source and the mask, and means for adjusting the pressure within the conduit to changeable target pressure forms a system with time lag. In even proportional assist ventilation, the patient will feel discomfort if the pressure of the respiratory gas is controlled at intervals of not less than 100 msec. However, open-loop control is not suitable for such a system due to the delay or inaccuracy of the control.